Genes that are crucial in the normal development of multi-cellular organisms often play a role in oncogenesis when aberrantly expressed in adult tissues. The Notch signaling pathway is crucial in the cell fate determination, and there is strong evidence demonstrating a role for Notch dysregulation in tumor development. Despite the increasing role of Notch pathway in human cancers, very little was known about the role of Notch3 in lung cancers. Our group was the first to link Notch3 pathway with lung cancers. We demonstrated that about 40% of resected lung tumors overexpresses Notch3. In the developing lung, constitutively activated Notch3 prevents maturation of lung epithelium. Furthermore, inhibiting the Notch3 reduces tumor phenotype, induces apoptosis and renders the tumor cells more dependent of exogenous growth factors. Finally, pharmacologic inhibition of Notch activation reduces proliferation of lung cancer in vitro. Based on our preliminary data, we hypothesize that the Notch3 signaling pathway plays a role in the pathogenesis of lung cancer and represents a potential target for intervention. To test these hypotheses, (1) we will examine whether Notch3 is sufficient for cellular transformation in vivo using an inducible, Clara cell-driven Notch3 expressing mouse model. (2) While transformation is an important aspect in cancer pathogenesis, understanding whether Notch3 is important in progression is also important. We will examine the effect of inhibiting Notch3 on tumor survival, progression and metastasis in established tumor using an orthotopic lung cancer model. (3) Gamma-secretase is a presenillin-containing protein complex necessary for proteolytic cleavage and activation of Notch receptors. In this aim, we propose to examine the phenotypic and biochemical effects of y-secretase inhibitors on tumor xenografts and to determine the degree to which the anti-tumor effect is Notch-related. These proposed studies will potentially identify Notch3 as a target for intervention and provide insights into the mechanism of Notch3-related lung cancer pathogenesis. [unreadable] [unreadable] [unreadable]